1. Field of the Invention
The present invention relates to an electronically controlled shift range switching apparatus that switches the shift ranges of an automatic transmission by driving an actuator and particularly to a shift range switching apparatus that can perform fail-safe control when a power source supplied to the shift range switching apparatus is instantaneously interrupted.
2. Description of the Related Art
In recent years, electronically controlled shift range switching apparatuses for switching the shift ranges of an automatic transmission by driving an actuator including an electric motor have been put to practical use. In such a shift range switching apparatus, there exists no mechanical coupling between a shift range setting means (in general, an input operation unit that is referred to as a “shift lever” or a “selector lever”) for a driver to perform shift range switching operation and a shift range switching mechanism (a mechanism for mechanically shifting in a sliding manner a manual spool valve that switches the hydraulic pressure circuits in an automatic transmission apparatus, in accordance with a shift range selected by the driver); therefore, the degree of flexibility in the vehicle mountability of the shift range setting means is raised. Moreover, the configuration of the shift range setting means can freely be designed; thus, a shift range setting means such as a “touch panel”, a “push button”, or a “jog lever”, which has a high operability in terms of human engineering, is widely utilized.
In such a shift range switching apparatus, in accordance with a demanded shift range that is outputted from the shift range setting means, due to shift range switching operation by a driver, the actuator is controlled in such a way that the real shift range coincides with the demanded shift range; therefore, in the control of shift range switching, it goes without saying that two information items, i.e., the demanded shift range (target shift range) and the real shift range (actual shift range), which is the result of control of the actuator, are extremely important.
In general, an electronically controlled shift range switching apparatus is actuated after receiving a power source from a vehicle battery; therefore, instantaneous power interruption (energization is instantaneously interrupted) may occur, due to some sort of unexpected cause (e.g., unexpected loose connection of electric wiring leads).
In a conventional mechanical shift range switching apparatus, a driver's operational force switches shift ranges, through a mechanical transfer mechanism such as a shift cable or a shift rod; therefore, as long as the mechanical coupling is not broken, the shift range (the target shift range) set through the driver's switching operation of the mechanical shift lever and the real shift range (actual shift range) realized through the operation of the shift range switching mechanism securely coincide with each other.
However, an electronically controlled shift range switching apparatus has no mechanical coupling between the shift range setting means and the shift range switching mechanism; therefore, after being actuated again due to instantaneous power interruption, the electronically controlled shift range switching apparatus may lose information indicating the target shift range or the real shift range and may be prevented from normally functioning.
Accordingly, for example, in Patent Document 1, fail-safe control for coping with the re-activation is proposed in which a storage (a nonvolatile storage that can maintain data even when the power source is shut down) that does not lose data even when the power is instantaneously interrupted is utilized and the output axle position of the actuator (the positional information related to the real shift range) is always stored, “so that when due to instantaneous power interruption, the shift range switching apparatus is actuated again, control is continued by use of the output axle position of the actuator stored in the nonvolatile storage, under the condition that the electric motor has not been driven before the instantaneous interruption of the power source” or “the range demand value to be outputted from the shift range setting means is set to the output axle position of the actuator stored in the nonvolatile storage and then control is continued, under the condition that the electric motor has not been driven before the instantaneous interruption of the power source”.
As far as the shift range setting means for a driver to switching the shift ranges is concerned, a momentary-type input device such as a “touch panel”, a “push button”, or a “jog lever” has predominantly been utilized in recent years. For example, Patent Document 2 discloses a shift range setting means utilizing a touch panel; when respective portions of the touch panel, at which D Range (forward travel range), N Range (non-travel range), R Range (retreat travel range), and P Range (non-travel range and parking lock operation) are displayed, are softly touched by a fingertip, the shift range selected by a driver is outputted, as the demanded shift range, from the shift range setting means.
Accordingly, it is required to electrically store at least the demanded shift range until after the demanded shift range is outputted by the shift range setting means, the actuator is controlled and then the real shift range coincides with the demanded shift range.
However, when due to instantaneous power interruption, the shift range switching apparatus is actuated again, information indicating the demanded shift range is lost and hence the shift range switching apparatus may not be able to function normally; thus, as is the case with Patent Document 1, described above, it is required to utilize a nonvolatile storage is utilized so that the information indicating the demanded shift range is constantly stored. As a result, even when due to instantaneous power interruption, the shift range switching apparatus is actuated again, the control utilizing the demanded shift range stored in the nonvolatile storage can be continued.